Fundamentación normativa

these authors is compared with coronagraph data and EUV disk observations from the SOHO satellite together with ground-based H a images. These data are evaluated in order to determine the accuracy o f the sigmoid-to-arcade proxy in CME onset detection. Two example eruptive regions are discussed in detail in order to examine the difference between the eruption o f a sigmoidal region thought to consist o f a single structure and a region thought to appear sigmoidal as a result o f projection effects acting on more than one feature. The implications o f this study for CME onset prediction using soft X-ray data are discussed.

The Canfield, Hudson and M cKenzie [1999] study, together with other recent observational studies o f sigmoidal features [e.g. Pevtsov, Canfield & Zirin, 1996; Aurass et al., 1999; Wang et al., 2000; van Driel-Gesztelyi et al, 2000; Gibson et al., 2002], focused primarily on active region sigmoids. To date, no detailed studies into the nature o f large scale sigmoidal features, unconfined by a single active region, have been carried out. Typical active region loop lengths are o f the order o f 10-100,000 km [Priest, 1982]. H owever, non-active region sigm oids may be several tim es longer than this upper value and can exist within regions o f quiet-Sun or in the vicinity o f diffuse flux systems, previously classified as active regions. Chapter 5 describes the formation and evolution o f a sigm oid that developed within the decayed remnants o f an active region. The

appearance o f this region is studied over the course o f three rotations leading up to the formation o f the large scale sigmoid. The region’s CME and flaring activity is also considered. CME activity is found to continue through all three rotations, even though flaring activity has almost ceased prior to the appearance o f the large scale sigmoidal feature.

1.4.2: Transequatorial Loops

Transequatorial loops were initially observed in Skylab im ages o f the solar corona [Chase et al., 1976; Svestka et al., 1977; Svestka & Howard, 1981]. Through analysis o f Yohkoh soft X-ray observations between 1992 and 1998 Pevtsov [2000] found transequatorial loop systems (TLS) to be a frequent feature o f the soft X-ray corona. This study identified several classes o f interconnecting loop. Observations included loops that appeared sheared in S (or reverse-S) configurations and loops that appeared to form a magnetic X-point separating active regions in either hemisphere and undergoing reconnection [Tsuneta, 1996].

The Pevtsov [2000] study focused primarily on the frequency and morphology o f transequatorial loop system s, rather than a connection between these loops and CME or flaring activity. Khan & Hudson [2000] observed a series o f hom ologous disappearances o f a transequatorial loop system. Each disappearance was associated with flaring and CME onset.

The results described in Chapter 6 o f this thesis form the first survey o f transequatorial loops in terms o f their eruptive character. This study follow s on from the Khan and Hudson [2000] study that considered a single eruptive transequatorial loop system. The study described in Chapter 6 considers a sample o f 18 transequatorial loop systems. Comparison o f soft X-ray TLS observations with white light coronagraph and EU V Fe XII (195 Â ) disk observations from the SOHO/LASCO and SOHO/EIT instruments is made in order to determine how frequently transequatorial loop systems are involved in CME onset. The results o f this study are then discussed in terms o f the implications for transequatorial loop systems in CME onset prediction.

Chapter 1. Introduction___________________________________________________________ ^

1.5 Summary

This thesis describes three com pletely new studies which aim to advance the understanding o f sigmoidal features and transequatorial loop system s observed in the corona and their association with eruptive solar activity. This is achieved by:

(1) A survey o f sigmoidal solar features in terms o f their probability o f eruption and association with CME onset. This study complem ents the previous work o f Canfield, Hudson and M cKenzie [1999] which classified sigm oidal features as eruptive in terms o f their appearance in soft X-ray data alone. The survey described in Chapter 4 incorporates data from both the Yohkoh Soft X-ray T elescope (SXT) and the Solar and Heliospheric Observatory (SOHO). Detailed studies o f two sample eruptive sigmoidal features are also included in this chapter.

(2) A detailed study o f the formation and evolution o f a large scale sigmoidal feature observed to form in the decayed remnants o f an active region follow ing CME onset. This constitutes the first study o f a feature o f this kind, the Canfield, Hudson and M cKenzie survey having considered only sigm oids that formed in young active regions, closely associated with strong magnetic fields.

(3) The first survey o f transequatorial loop systems in terms o f their association with CME onset. This survey extends the results o f Khan & Hudson [2000]. These authors found transequatorial interconnecting loop system s to be associated with CME onset in a single case study. The study described in this thesis extends this result to a list o f 18 transequatorial loop system s (TLS), providing comparison between Yohkoh SXT and SOHO data in order to determine what proportion o f the TLS are associated with CME onset.

The follow ing Chapters 2 and 3 describe the instrumentation and data extraction techniques em ployed throughout this thesis. A description o f each study can be found in Chapters 4, 5 and 6 respectively, and final conclusions in Chapter 7.

Chapter 2: